1. Field of the Invention
This invention relates to a method of processing end portions of optical fibers utilized in, for example, optical communications, and more particularly to a method of processing end portions of optical fiber elements which elements are well suitable for end-to-end connecting together and also relates to such optical fibers having end portions specifically processed.
2. Description of the Related Art
FIGS. 19 and 20 show the configuration of the end portion of an optical fiber to be connected to a conventional optical connector. In the drawings, the reference numeral 10 indicates the optical fiber and 20 the ferrule bonded to the end portion of the optical fiber 10. The ferrule 20 is of a hollow cylindrical shape and has a through-bore 21 formed through its center axis for receiving an optical fiber element 11 which is a bare fiber exposed by removing away a protective coating therefrom. The fiber element 11 is inserted into the through-bore 21 and fixed or adhered thereto by adhesive 20A.
In a conventional optical connector, the end terminal face of the ferrule 20 having an optical fiber element 11 adhesively fixed thereto is ground to a convex spherical shape, and a pair of such identical ferrules 20 thus shaped are then brought into end-to-end abutment and joined together within a split sleeve 30 (see FIG. 21). For this process, the PC (Physical Contact) joining method is used which involves applying urging pressure to the end faces of the pair ferrules 20 by resilient springs (not shown) to elastically deform the core of the respective optical fiber elements 11 of the optical fibers 10 lying at the apices of the convex spherical ends. With this PC joining method, no air space is produced between the optical fiber elements 11, allowing for the joining at a low transmission loss.
Currently, however, optical connectors configured so as to connect optical fiber elements 11 directly together without the use of the ferrules 20 have been designed in view of the demand for more compactness and finer pitches of optical connectors. But, the optical connector of such configuration still requires the PC joining method in order to accomplish the purpose of reducing the loss. Further, the urging pressure for effecting the PC joining in this type of optical connector is characterized in that it is generated by axially compression-deforming the optical fiber elements 11 and utilizing the restoring force (which will be hereinafter referred to as buckling load) from the compressive deformation.
In this regard, the magnitude of the buckling load generated by compression-deforming the optical fiber elements 11 is on the order of 0.2-0.4N. Depending on the condition of the end face of the optical fiber, particularly if the end face has been cut at an angle θ which is not a right angle with respect to the fiber axis as illustrated in FIG. 22A, even a buckling load TH exerted on such fiber as shown in FIG. 22B may fail to sufficiently compression-deform the opposed cores, resulting in occurrence of a gap G between the opposed fiber ends as shown in FIG. 22C and hence inability to accomplish the PC connection. Consequently, it is undesirably difficult to achieve stable optical properties.
One approach currently proposed to solve this difficulty is to form the end portion of an optical fiber in a shape of a convergent taper by using the technique as disclosed in the Japanese Patent Publication Kokoku 3-50246, and then cut the tapered end to obtain a flat end face, thereby to optically couple a pair of the thus obtained optical fibers together by butt-joining the flat end faces.
This method allows for facilitating the deformation of the opposed fibers at their extreme cut ends to secure good optical coupling result even if the end faces are cut more or less at an angle θ, because the end faces to be abutted together are reduced in area due to the convergent taper.
Nevertheless, the thus obtained optical fiber elements 11 have a drawback that the optical fiber elements 11 are vulnerable to failure due to their reduced mechanical strength when they are subjected to the connecting method as mentioned above by abutting them against each other and subjecting them to buckling load.
In order to overcome this drawback, there was an approach toward providing the peripheral surface of the optical fiber element 11 with a coating film 11C of carbon, resinous material or the like as illustrated in FIG. 23.
However, when an attempt is made to form a taper end portion at its terminal end of such optical fiber element 11 covered with the coating film 11C, by using the etching technique in accordance with the method as disclosed in the Japanese Patent Publication Kokoku 3-50246, the etching process would start with the end face of the optical fiber element 11 which is only the portion exposed from the coating film 11C with the peripheral surface of the cladding 11B of the optical fiber element 11 being covered with the coating film 11C, so that the etching would proceed from the core 11A located in the center of the fiber element 11, with the result that the optical fiber element 11 would be etched in a generally cylindrical form, and thus end in failure to form tapered surface portions.
For this reason, the present inventors endeavored to solve this drawback in the technique of forming a taper on an optical fiber element 11 coated with a coating film 11C, and conceived such a technique as to deposit a resist film 13 on the end terminal surface of the optical fiber element so as to cover the entire end face of the core 11A and the radially inner half part of the cylindrical end face of the clad 11B as shown in FIG. 24 prior to effecting the etching process and then dipping the end portion of the fiber element thus covered with the resist film 13 into an etching solution J as shown in FIG. 25.
According to this endeavored method, since the end face of the core 11A is fully protected from the etching by the resist film 13, it was found that the immersed end portion of the fiber element is formed with a reduced-diameter portion 14 extending upward from its extreme end.
It was also found that the fiber element was provided with a tapered surface portion TP at the upper part of the reduced-diameter portion, that is, a part of the fiber element corresponding to ultimately at an elevated portion from the liquid level of the etchant J.
After a desired tapered surface portion TP has been obtained, those portions of the coating film 11C corresponding to the reduced-diameter portion 14 and the tapered surface portion TP are removed.
However, this endeavored method still has the disadvantage that it requires an additional step of applying a resist film 13 to the end face of an optical fiber element 11 prior to forming a tapered surface portion thereon, and also another additional step of removing the coating film. It makes thus the manufacturing process correspondingly cumbersome. Particularly in the case of an optical fiber having a multiplicity of optical fiber elements 11 integrally incorporated therein such as the flat tape type optical fiber, the operation of depositing a resist film 13 to the end face of each individual optical fiber element 11 has proved too cumbersome to put this method into practical use.
Turning now back to the prior art of Japanese Patent Publication Kokoku 3-50246, when the optical fiber element is cut directly on the tapered surface portion as disclosed this prior art, the cutter edge may slip axially along the angular tapered surface portion to exert an axial force on the fiber element during the cutting process. Consequently, this method has another drawback that flaws such as cracks may possibly occur in the cut portions.
In addition, in order to form tapered surface portions on the individual fiber elements 11 of a tape type optical fiber array 10T comprising a plurality of optical fiber elements 11 held by a tape-like sport 12T as illustrated in FIG. 26, the Japanese Patent Publication Kokoku 3-50246 also discloses a method involving dipping the individual fiber elements 11 in an etchant J. As one example, it is disclosed that even if the fiber elements immersed in the liquid are not equal in length, a plurality of tapered surface portions which are shaped in conical terminal ends are uniformly formed in their lengths, since those portions of the fiber elements immersed in the liquid should be completely dissolved by dipping them for a sufficiently longer period of time (such as, 30-60 minutes) which is enough for the complete dissolve.
The present inventors have discovered, however, when the above-identified prior art technique were applied to a tape type optical fiber array 10T comprising a multiplicity of individual fiber elements 11 having as fine an array pitch PN as around 0.25 mm so as to accomplish the purpose of the present invention, a specific phenomenon occurred that the levels of the respective liquid heads of the etchant which heads are adhering to the respective associated fiber elements, could rise higher as it is closer to the middle of the array due to the surface tensions acting on the respective liquid heads adhering to individual fiber elements 11 overlapping to each other (see FIG. 27).
As a result, the troubles occur that the axial dimension (length) L of the tapered surface portion TP formed by etching gets successively longer in the order of L1<L2<L3<L4 toward the middle of the array of the fiber elements 11 and that the positions of the tapered surface portions are displaced with respect to each other. Especially, the axial dimensions L1<L2<L3<L4 of the tapered surface portions TP tend to be longer than that obtained when the etching is conducted on a single fiber.
By way of example, if the array pitch PN of the fiber elements 11 is 0.25 mm and the diameter of the fiber elements 11 is 0.125 mm (125 μm), the axial dimensions L1<L2<L3<L4 of the tapered surface portions TP will be around 0.3 to 0.5 mm. On the other hand, when the etching is conducted on a single fiber, the axial dimension of the tapered surface portion TP will be around 0.1 mm. This means that the axial dimension L of the tapered surface portion TP formed on the multiple-fiber type optical fiber array will be about 3 to 5 times as long as that of the single-core fiber.
As the axial dimension L of the tapered surface portion TP increases, the strength of the tapered surface portion TP adjacent its forward end correspondingly decreases. With repeated connecting operations of optical connectors, fatigue is built up in the tapered surface portion TP, causing a durability problem.
As discussed above, the conventional methods and the prior attempt conducted by the present inventors are still insufficient due to various problems. Accordingly, there has been a need for obtaining an effective and practical method for processing end portions of optical fibers as well as optical fiber having an improved end portion which is applicable to the repeated connection purpose.